Yet despite all the hype, some recent studies in both the US and the UK have thrown up some alarming findings: that the treatment can trigger mania, depression, hallucinations and psychosis in some people.

Transcendental meditation is the most popular form among celebrities and CEOs alike, and is described as a “simple, natural, effortless technique practised 20 minutes twice each day while sitting comfortably with the eyes closed.” (Post continues after gallery.)

Celebrities who meditate

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For many people, meditation and mindfulness are ways to block out the outside world and focus in on ourselves. As a result of the intense concentration and deep thinking, it can force us to confront our worries and fears that everyday activities like watching TV, being surrounded by people, eating and drinking allow us to ignore.

A US study by David Shapiro, a professor at UCLA Irvine, looked into the effects of meditation retreats. Examining 27 people with varying levels of meditation experience, he found over 60 per cent had suffered at least one negative side-effect, including panic, depression, anxiety, pain or confusion, after attending one of these retreats.

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Seven per cent had experienced profoundly adverse effects.

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While it's certainly true that several days of meditation, as would be experienced on a retreat, would be overwhelming to someone who was new to the practise, Dr Shapiro found that the amount of experience made no difference. In fact, an early study had even found those with more experience had had considerably more adverse effects than the beginners.

Life coach Stephanie Kakris says the emotions that come with meditation can feel overwhelming at first.

"As children we are told to 'calm down', 'don't cry' and 'there's nothing to be scared of'. We are taught that our emotions are wrong. When we start feeling them as adults it's no wonder it can be scary," she says.

"This is where the potential adverse effects of a meditation retreat can come in, if the education or support is lacking with respect to feeling the emotions. However, if handled correctly, and we allow ourselves to feel the fear, have a good cry or express some anger out in a safe environment, incredible peace and calmness can be felt."

In some situations, this can be beneficial. But studies suggest it could also cause the very thing it tries to cure. So is meditation really for everyone?

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According to counsellor and mindfulness practitioner Rachel Davey, the clear advice for mindfulness meditation (or Mindfulness Based Cognitive Therapy MBCT, which has a very solid evidence base on the prevention for relapse of depression) is that it should not be practised while you are currently experiencing clinical depression. (Post continues after gallery.)

Famous women talk about their mental health

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Catherine Zeta Jones.

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"It's best to wait until you have received treatment for a currently depressive episode, and use MBCT to prevent relapse," Davey says.

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"This is for a variety of reasons, not least of which that you need a degree of insight into your own thoughts and internal processes when meditating, and if you are suffering from active psychosis or suicidal ideation, it's probably not a good idea to start to examine those thoughts in greater depth right now."

"For people who are basically healthy, or if you have a tendency for mental health issues, if these are well managed, then mindfulness meditation has proven to have a number of benefits for both mental health as well as physical health and wellbeing," she says.

According to researchers, while the media is full of positive reports about meditation, the problem is the shortage of studies into the other side.

"It's hard to have a balanced view when the media is full of articles attesting to the benefits of meditation and mindfulness," Catherine Wikholm, a researcher in clinical psychology at the University of Surrey, writes in The Guardian.

"We need to be aware that reports of benefits are often inflated...whereas studies that do not discover significant benefits rarely pick up media interest, and negative effects are seldom talked about.

Media tends to focus on the benefits. Image via iStock.

Miguel Farias, head of the brain, belief and behaviour research group at Coventry University, believes the shortage of rigorous statistical studies into the negative effects of meditation was a 'scandal'.

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"The assumption of the majority of both transcendental meditation (TM) and mindfulness researchers is that meditation can only do one good," he told The Times.

"This shows a rather narrow-minded view. How can a technique that allows you to look within and change your perception or reality of yourself be without potential adverse effects?"

"The answer is that it can't, and all meditation studies should assess not only positive but negative effects," he says.

The pair were part of a team which conducted a study measuring the effect of yoga and meditation on prisoners, with the findings recently published in the psychologists' book, The Buddha Pill: Can Meditation Change You? (Post continues after video.)

Inmates at seven prisons took 90 minute classes each week and had to complete tests to measure their higher cognitive functions across the ten-week randomised control trial.

The study found that while the prisoners' moods improved and their stress and psychological distress was reduced, they were found to be just as aggressive as before the mindfulness techniques.

But does that undo all the positive benefits mindfulness and meditation can give?

Research Manager at the University of Sydney Genevieve Baij says definitely not.

"Preliminary research suggests that there are different effects pending on the context of the person. But while there are definite benefits, we need to do more research into the limits of these benefits — who it's good for and who it's not," she says.